Why doesn't my android start daydreaming?

In Settings > Display > Daydream I have - selected one (any one, but right now it's just the standard **Clock**) - when to daydream = either docked or charging - "start now" works fine. I've connected my Nexus 4 by USB cable to a wall charger and "Clock" confirms *Charging (AC), 45%* Settings > Display > Sleep is set to 15 seconds while I try to get daydreaming to work. If/when it works, I'll probably set it to 2 minutes. Despite the above settings, and being on AC power, **the phone never daydreams. Why not?** This is a Nexus 4 with stock 4.2.2, not rooted, not superuser'ed (yet).